/**
* @author AlbertDarren
* @contact 2563491540@qq.com
*/
#include <cstdio>

using namespace std;

int main_27()
{
	int n,sum=0;
	scanf("%d",&n);
    do
    {
        sum+=n--;
    } while (n);
    printf("%d\n",sum);
	return 0;
}
